ELECTRIC (JOULE) HEATERS

An electric (Joule) heater is a device in which heat release in an electrical conductor due to electric current flow (Joule Heating) is used for heating solid, liquid and gaseous substances. It is more convenient to call such devices electric furnaces, in which electrical heaters are the heat release units. Electric furnaces have the following main advantages compared with burner furnaces:

The possibility of concentrating large releases of heat energy in a small volume.

The possibility of providing high heating uniformity, or given nonuniformity, by appropriate arrangement of heat sources (heaters).

The relative simplicity of electric power control (consequently, temperature) and the automation of the electric furnace’s temperature regime.

The convenience of mechanization and automatization of product or materials charging/discharging processes that substantially facilitate the inclusion of electric furnaces in processing assemblies.

Electric furnaces can be easily sealed, preventing the oxidization of heated products or materials by using a protective atmosphere or vacuum.

Depending on the heating method, electric furnaces are divided into two types: indirect heating furnaces, in which heat is released from a heater and then transferred to a heated body by convection and/or thermal conduction, and direct heating furnaces, where a body is directly connected in an electrical circuit and is heated by current flowing through it. Most electric furnaces are of the first type. In the case of convective heating, heat transfer occurs by gas, liquid substance (liquid metal, molten salt, etc.) or by fluidized bed (heat from the heater is transferred to a processed product by numerous fine hard bodies, which are in continuous motion and in contact with the heater and the heated product). Indirect electric furnaces are divided into two types; namely, electric furnaces for thermal processing of products and materials and electric furnaces for melting.

Electric furnaces are also classified according to:

Operating time, either periodic or continuous (methodical) electric furnaces;

Operating temperature. Under this classification are low-temperature electric furnaces (less than 1,000 K) in which a major part of the heat is transferred to a product by convection; moderate-temperature electric furnaces (up to 1,700 K) where heat transfer is predominantly by radiation; high-temperature electric furnaces (more than 1,700 K) which include furnaces that cannot operate with metal heaters without a protective atmosphere or vacuum).

Atmospheric volume during operation. Electric furnaces may be furnaces with oxidizing medium (usually air), with controlled atmospheres or with vacuum.

The presence in electric furnaces of zones with high temperatures requires, on the one hand, materials which can withstand such temperatures and, on the other, materials which provide heat insulation for the zones from both the other parts of the electric furnace and the ambient medium. As applied in electric furnaces, such materials are divided into the following types: 1) the refractory; 2) heat-insulating; 3) heat resistant; and 4) materials for heaters.

Refractory materials

Refractoriness is the property of a material which enables it to withstand high temperatures. It is determined by comparing the deformation of a material undergone after heating with a special standard geometry test specimen and is measured in K. Besides refractoriness, materials must have sufficient thermal stability, i.e., the capability to sustain without failure a sufficient number of sharp thermal changes.

Refractory materials must be durable at temperatures near the heating source, be sufficiently strong and have a low thermal conductivity. The last requirement often is not provided, that is why thermal protection (so-called lining) of moderate- and high-temperature electric furnaces consists of at least two layers: refractory and thermal protection. The second layer is free from mechanical loading, but limits thermal losses to an acceptable level. In case of excessive mechanical loading of a refractory material, support mountings made from heat-resistant steel are provided.

Refractory materials are based on the application of the following oxides: SiO2 (silica, refractoriness up to 2,000 K); Al2O3 (alumina, up to 2,100 K); MgO (magnesia, up to 3,100 K). The most widely used in electric furnaces are refractory products made of chamotte (formed from clays with large amounts of alumina). This material is sufficiently heat-resistant, has a relatively small linear thermal expansion coefficient and can sustain sharp temperature fluctuations. The critical operating temperature of chamotte is 1,700 K.

Another widely-used refractory material is dinas (made of quartzite). The distinct feature of dinas is its mechanical strength at high temperatures. The disadvantage is its inclination to crack during sharp temperature changes. The critical operating temperature of dinas is 1,900 K.

Magnesia-based refractory materials have good heat-resisting qualities but do not have sufficient strength and thermal stability at high temperatures.

In low- and moderate-temperature electric furnaces, porous refractory products are often used, particularly light chamotte and foam chamotte. They have less refractoriness and mechanical strength, but have much less thermal conductivity compared with the usual chamotte; therefore, they can be simultaneously used as refractory and heat insulating materials.

Heat-insulating materials

One of the most widely-used heat-insulating material is diatomite. It is composed of dried-up remains of very fine aquatic plants (diatom), consisting mainly of SiO2 and pierced through by extremely fine pores which cause low density and thermal conduction. The maximal operating temperature of diatomite is 1,200 K. It is used in the form of bricks or as a fill.

Asbestos and derivative products are also good heat insulators. Its maximal operating temperature is up to 800 K. Asbestos, however, has a high hygroscopicity and presents potential health hazards. Heat-insulating wools (glass, mineral, etc.) also enjoy wide usage.

Lining design and the accuracy of its performance have an influence on heat loss magnitude, electric energy consumption, run-up time, life span, weight, overall dimensions and cost of an electric furnace. A lining is characterized by a value of accumulated heat, Q, during electric furnace run-up to a stationary regime, and a value of heat loss through the lining to an ambient air, W. A choice of lining (materials and thickness) is made by comparing these two values over a continuous operating time, t. One of the possible criteria for the optimal choice of a lining is a minimal value of Q/t + W. Lining thermal calculation is performed taking into account the temperature dependence of thermal conductivity.

Heat-resistant materials

These materials must meet the following requirements: 1) Sufficient heat resistance, i.e., an ability to endure prolonged operation at high temperatures without substantial surface oxidizing; 2) Sufficient high-temperature strength, i.e., an ability to maintain a mechanical strength at high temperatures; 3) Sufficient high-creep limit, i.e., small residual deformations under mechanical loading at high temperatures; 4) Characteristic stability of heaters during prolonged operations; 5) Adaptability to machining and welding for the production of various parts and devices.

The main heat-resistant materials are metals, in particular steel, because they are adaptable and have sufficient mechanical strength and a relatively low creep. Chrome-nickel steel is the most widely used in electric furnaces.

Materials for heaters

Materials for heaters must have the following qualities:

Heat-resistance;

Sufficient high-temperature strength, providing an absence of heater deformation under its own weight during operation;

High electric resistivity. If this is not the case, the length of heaters is relatively large and difficulties of its placement in electric furnace appear;

Low temperature coefficient of electric resistivity. For practically all heater materials, this coefficient is positive; therefore during electric furnace run-up, the consumed electric current can exceed its rated value by more than 10 times. This fact has to be taken into account when designing an electric furnace power system. In some cases during the electric furnace run-up period, it is necessary to lower the voltage;

Characteristic stability of heaters during operations. For some materials, electrical resistivity increases over time, therefore the consumed power decreases. If it is necessary to sustain a constant power, then a controlled step-up transformer is used;

Constant size. Some heaters have high thermal expansion and this leads to structural and operational inconveniences;

Adaptability to heater fabrication processes. It is necessary to make metal wires and strips for heaters. It is also desirable to have a knowledge of their welding. For nonmetal materials, it is necessary to mould and press them to have heaters of a required shape.

Materials for heaters which satisfy the above requirements to a great extent are nickel-chrome alloys (nichromes). The heat-resistance of nichromes is due to their very strong films of chrome oxide Cr2O3, which has a higher melting temperature than the alloy and endures well cyclical thermal loads. Nichromes can withstand operational temperatures reaching 1,500 K. Ferrochromenickel and ferrochromealuminium alloys are cheaper than nichromes but have operational disadvantages, such as brittleness, low mechanical strength, etc. In high-temperature electric furnaces, heaters made of molybdenum, tungsten, tantalum and niobium are used, but due to their intense oxidation in air they require a protective atmosphere.

Among nonmetallic materials for heaters, the most widely-used is silit. It is fabricated from carborundum (silicide of carbone, SiC). The heaters are made in the form of rods and tubes. Their operating temperature reaches 1,800 K. For operations in an oxidizing atmosphere with temperature of up to 2,000 K, heaters made of disilicide of molybdenum are used. In electric furnaces with a neutral atmosphere or a vacuum, heaters made of carbides of refractory metals (zirconium, niobium, tantal, hafnium) are used. The most refractory among them is hafnium carbide (Tmelt = 4160 K).

Metal heaters are usually fabricated in the form of wire spirals and zigzags (made from wire or strip). Also in abundance are closed-tube heaters. Such heaters are made in a form of a tube of high-temperature steel, with the heaters (usually of spiral form) located at tube axes and having hermetically-sealed leads. The space between the spirals and the tube wall is filled by materials with high thermal conductivity and good electric insulation. The main advantage of a tube heater is the absence of electric potential on the tube surface, which ensures safe operation where the electric feed is from a standard electric network, for example, in domestic heating devices. Nonmetal heaters are usually made in the form of rods and tubes.

Vacuum electric furnaces

Use of a vacuum instead of an inert medium is often more economic. Moreover, many technological processes and scientific researches can be conducted only in a vacuum. The main disadvantage of vacuum electric furnaces is their high cost (including vacuum equipment). Heaters for vacuum electric furnaces are usually made of the same materials as those for electric furnaces. In the case of electric furnaces with a relatively rough vacuum, it is necessary to take into account a decrease in electrical strength of the residual gas, according to the Paschenig law.